Gorilla sessions not working for CORS from client

I have set up a Go rest api. And on login I do this:

session, _ := store.New(r, sessionId)
session.Options.MaxAge = 12 * 3600
err := session.Save(r, w)
//treat error

and for checking the session i have smth like this:

	session, err := store.Get(r, sessionId)
    //treat error
	if session.IsNew {
		http.Error(w, "Unauthorized session.", http.StatusUnauthorized)
		return
	}

If I do the requests from postman it works fine, but when I do them from my client I get 401. Has any of you experienced something like this? The store is a CookieStore.

I already checked the id's, I replaced sessionId variable with a static string. Gorilla session uses gorilla context to register a new request and when I do the request from postman context.data[r] is not null, but from the client it is always null -> always a new session.

https://github.com/gorilla/context/blob/master/context.go - line 33

it is called in

https://github.com/gorilla/sessions/blob/master/sessions.go - line 122

wich is used in the CookieStore.Get function in

https://github.com/gorilla/sessions/blob/master/store.go - line 77

EDIT 1:
For the client I use polymer and I tried xmlhttp too.
Polymer:

<iron-ajax
  id="ajaxRequest"
  auto
  url="{{requestUrl}}"
  headers="{{requestHeaders}}"
  handle-as="json"
  on-response="onResponse"
  on-error="onError"
  content-type="application/json"
  >
</iron-ajax>

and the handlers

  onResponse: function(response){
    console.log(response.detail.response);
    this.items = response.detail.response
  },
  onError: function(error){
    console.log(error.detail)
  },
  ready: function(){
    this.requestUrl = "http://localhost:8080/api/fingerprint/company/" + getCookie("companyId");
    this.requestHeaders = {"Set-cookie": getCookie("api_token")}
  }

and the cookie successfully reaches the backend.

And xmlhttp:

  var xmlhttp = new XMLHttpRequest();
  xmlhttp.onreadystatechange = function() {
    if (xmlhttp.readyState == XMLHttpRequest.DONE ) {
      if(xmlhttp.status == 200){
        //do stuff
      }else if(xmlhttp.status == 401){
        page.redirect("/unauthorized")
      }else{
        page.redirect("/error")
      }
    }
  }

  xmlhttp.open("GET","http://localhost:8080/api/fingerprint/company/" + getCookie("companyId"),true);
  xmlhttp.setRequestHeader("Set-cookie", getCookie("api_token"));
  xmlhttp.send();

EDIT 2:

So I tried debugging with fiddler(thanks for the suggestion) and i found out that the request from postman has an bold entry Cookies / Login and the request from the client does not. Any idea how to get/set that value? It is somehow automatically set in Postman. In the authentication request I get a set-cookie header that has all the data that I need but I can't get it on the client. I get Refused to get unsafe header set-cookie.

The problem is that in the client the requests need to have withCredentials = true and after that the browser deals with everything. It gets the cookie from the set-cookie header and it sends the cookies via the cookie header. So, after all, it was not a gorilla sessions problem.

If anyone else is having the same problem I was having and you want to whitelist all domains/wildcards (or have a list of domains in an array you can scan through), you can do something like this.

domain_raw := r.Host
domain_host_parts := strings.Split(domain_raw, ".")
domain := domain_host_parts[1] + "." + domain_host_parts[2]
domains := getDomains() // stores a slice of all your allowable domains
has_domain := false
for _, d := range domains {
	if d == domain {
		has_domain = true
		break
	}
}

if has_domain == false {
	return
} else {
	w.Header().Add("Access-Control-Allow-Origin", "https://"+domain_raw)
	w.Header().Add("Access-Control-Allow-Credentials", "true")
}

I love go